Design of planar wide bandpass filters from wideband planarantennas

Author

Abbosh, A, Bialkowski, M, Thiel, D

Published

2009

Conference Title

2009 IEEE ANTENNAS AND PROPAGATION SOCIETY INTERNATIONAL SYMPOSIUM ANDUSNC/URSI NATIONAL RADIO SCIENCE MEETING, VOLS 1-6

DOI

https://doi.org/10.1109/APS.2009.5172264

Copyright Statement

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Design of Planar Wide Bandpass Filters from Wideband Planar Antennas

A. Abbosh1, M. Bialkowski1

, and D. Thiel2

lSchool ofITEE, The University of Queensland, Qld4072, Australia{abbosh,meb}@itee.uq.edu.au

2Griffith School of Engineering, Griffith University, Qld4III, Australiad.thiel@griffith.edu.au

Introduction

Wideband bandpass filters (BPF) are the key components of modem widebandcommunication systems. Due to the requirements of low cost and easy integration withthe other circuit elements, planar filters have attracted most of the attention.

There are varieties of planar bandpass filters with wideband performance which haverecently been proposed and investigated for use in wideband communication systems. Forexample, planar BPFs using end-coupled coplanar waveguides (CPW) were introduced in[1]. BPFs based on the combination of CPW lowpass and highpass periodic structureswere described in [2]. In order to obtain tight coupling in a wideband BPF, the use of aparallel-coupled microstrip line with a slotted ground plane was postulated andinvestigated in [3]. An alternative approach to overcoming the manufacturing problemspresent in edge-coupled planar structures was demonstrated in [4], where broadside­coupled structure was used to design ultra wideband BPFs.

Another type of wideband filters constructed by mounting a microstrip line in a lossycomposite substrate so as to attenuate the signals at high frequencies was described in [5].A wideband BPF employing two stopbands of a filter block with two tuning stubs on aring was presented in [6]. A compact UWB bandpass filter using a single multimoderesonator on microstrip line was reported in [7], whereas the design of a broadband BPFwith a short-circuited CPW multimode resonator was given in [8].

In some BPF designs a particular attention is paid to improving the out-of-band rejection.Examples include a modified class of multimode resonator-based wideband filters withimproved out of band rejection skirt [9] and harmonic passband suppression byemploying electromagnetic bandgap periodic structures [10].

In this paper, we explore the idea of designing wideband BPFs by utilizing two identicalwideband antennas. The tapered slot antenna is used for this purpose. In the proposedfilter, two compact tapered slot antennas with CPW feedlines are connected in the face­to-face configuration to construct a BPF. The success of the proposed concept isdemonstrated via full-wave electromagnetic simulations.

Design

The utilized antenna for the design of a wideband filter is shown in Fig. Ia. It belongs to aclass of tapered slot antennas (TSA), which have attracted a significant attention due totheir wideband performance, high directivity, low cost and easy integration with the radiofrequency circuitry [11]. An inherent feature of TSA is that it has a natural low frequencycut-off. This cut-off frequency is determined by size of the radiating aperture which isapproximately half-wavelength at that frequency. Otherwise, TSA has high-passproperties over ultra wide frequency band. Therefore to obtain a sharp high frequencycut-off, suitable means have to be devised.

978-1-4244-3647-7/09/$25.00 ©2009 IEEE

The TSA chosen in the present investigations has a linear slot profile. It is fed from acoplanar waveguide having broadband characteristics. The antenna's length (I) and width(w) are chosen following the guidelines presented in [11] and then optimised using thefull-wave electromagnetic simulator CST Microwave Studio. To maintain a wideimpedance bandwidth, the inner conductor of the feeder is tapered linearly in the mannershown in Fig. Ia. Assuming the use of Rogers RT60IO (Er = 10.2, thickness= 0.64mm) asa substrate, dimensions of the antenna are: L=I5mm, and W=30 mm. For the 50n CPWfeeder, the dimensions are W.F 2 mm, and S= 0.5 mm.

To form the proposed BPF, two identical TSA antennas are connected in the face-to-faceorientation as depicted in Fig. 1b. The overall dimension of the filter is 3cmx3cm. Theinput and output ports of the filter are coplanar waveguides.

In order to minimize the insertion loss of the designed filter at the passband, the structureshown in Fig. 1b is enclosed in a metallic box. This configuration is used to prevent anypossible radiation from the two antennas constituting the filter. Dimensions of the boxare: width=length=3 em and height=2 em. The filter's board is located at centre of thebox.

Results

Performance of the proposed filter is verified using the commercial software packageCST Microwave Studio.

Fig. 2 shows the simulated results for the return and insertion losses of the BPF. Theresults shown in Fig. 2 indicate that the device has a wide passband, which extendsbetween 5.1 GHz and 8.9 GHz assuming the 3 dB insertion loss as a reference. It isworthwhile to mention here that the parametric analysis that we carried out indicates thatthe passband location and value can be controlled by the antenna's dimensions (L and W).

The insertion loss of the designed filters at the centre of the passband is less than 0.2 dB,whereas the return loss is higher than 20 dB. The low cut-off band (below 5 GHz) issharp, providing 12 dBI GHz roll-off. However, the high frequency band (especiallyabove 11 GHz) has a slow cut-off rate. This is due to the high-pass behavior of thetapered slot antennas. To improve performance of the filter, the internal boundary of thestructure is corrugated in the manner shown in Fig. 1b. Lengths of the used corrugationsare chosen to be around half of the effective wavelengths at the high cut-off band (11GHz) to enable them to behave as a high band rejection filter. Dimensions of thecorrugations were then optimized to achieve the best possible cut-off for the high bandwithout jeopardizing the performance at the passband. The final values for thedimensions are: Length of the corrugations =1 mm to 3 mm, the width= 0.5 mm and theseparation between them=0.5 mm. Performance of the modified device is shown in Fig.2. It is apparent that the modified structure has a better high cut-off band with 20 dBIGHz roll-off, while the good performance at the passband and the low cut-off band ismaintained. The passband for the corrugated structure extends between 4.8 GHz and 8.5GHz.

The simulated group delay for the designed filter (with corrugations) is depicted in Fig. 3.It is clear from this result that the device has a group delay which has a very low peak-to­peak variation of less than 0.05 ns across the passband of the filter. Therefore, thedesigned filter shows attractive characteristics for wide BPF applications.

References

Outputport

Antenna 1 I Antenna 2...------------...------------

W Inputport

[1] D. Williams, and S. Schwarz, "Design and performance of coplanar waveguidebandpass filters," IEEE Trans. Microwave Theory Tech., vo1.31, pp.558-566, 1983.

[2] C. Hsu, F. Hsu, and J. Kuo, "Microstrip bandpass filters for ultra-wideband (UWB)wireless communications," IEEE MTT-S Int. Dig., pp. 679-682, 2005.

[3] M. MandaI ,and S. Sanyal, "Compact wideband bandpass filter," IEEE Microw.Wireless Compon. Lett., vol. 16, no. l,pp.46-48, 2006.

[4] A. Abbosh, "Planar bandpass filters for ultra wideband applications," IEEE Trans.Microwave Theory Tech., vol. 55, no. 10, pp. 2262-2269, 2007.

[5] A. Saito, H. Harada, and A. Nishikata, "Development of band pass filter for ultrawideband (UWB) communication," Proc. IEEE Conf. Ultra Wideband SystemsTechnology,pp.76-80,2003.

[6] L. Hsieh, and K. Chang, "Compact, low insertion-loss, sharp-rejection, and wide-bandmicrostrip bandpass filters," IEEE Trans. Microwave Theory Tech., vol. 51, no. 4, pp.1241-1246,2003.

[7] L. Zhu, S. Sun, and W. Menzel, "Ultra-wideband (UWB) bandpass filters usingmultiple-mode resonator," IEEE Microw. Wireless Compon. Lett., vol. 15, no. 11, pp.796-798, 2005.

[8] J. Gao, L. Zhu, W. Menzel, and F. Bogelsack, "Short-circuited CPW multiplemoderesonator for ultra-wideband (UWB) bandpass filter," IEEE Microwave WirelessCompon. Lett., vol. 16, no.3, 104-106, 2006.

[9] N. Thomson, and J. Hong, "Compact ultra-wideband microstrip/coplanar waveguidebandpass filter," IEEE Microw. Wireless Compon. Lett., vol. 17, no.3, pp.14-186,2007.

[10] J. Garcia-Garcia, J. Bonache, and F. Martin, "Application of electromagneticbandgaps to the design of ultra-wide bandpass flters with good out-of-bandperformance," IEEE Trans. Microwave Theory Tech., vol. 54, no. 12, pp. 4136-4140,2006.

[11] A. Abbosh, M. Bialkowski and H. Kan, Planar tapered slot antennas, Chapter six inPrinted Antennas for Wireless Communications Handbook, edited by R. Waterhouse,Wiley Int., USA, 2007.

I

(a) (b)

Fig.! The tapered slot antenna (a), and the filter composed of two corrugated antennas (b).

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